1. Field of the Invention
This invention relates to a transmitter for optical data communication. More particularly, this invention relates to a light emitting diode transmitter circuit with temperature compensation and with an input which is compatible with emitter coupled logic.
2. Description of Prior Art
In the present state of the art, optical data communication systems require transmitters which can convert electrical analogue or logic signals into lightwave signals. Commercially available emitter coupled logic (ECL) integrated circuits can operate up to the range of 200 to 300 megabits per second. This range is matched by the operating range of light emitting diodes (LED's). The electronic circuit which converts the logic or analogue input signal into a modulated current flowing through the LED is called a driver. For effective optical data communication the optical signal of the LED is required to be modulated with large amplitude, thus the driver is required to provide a large modulation of current through the LED.
Many applications require operation of the transmitter in wide temperature ranges. For example, in military applications operation at temperatures of -55 to 125 degrees Celsius are often required. However, the intensity output of LED's is sensitive to temperature variations such that intensity decreases as temperature increases. The effect of temperature increase can be compensated by increasing the current flowing through the LED, thereby restoring the optical intensity. For applications where the temperature can vary but the transmitter output signal must not vary in amplitude, the driver circuit must be supplemented by a circuit which senses the temperature increase and causes the LED current to increase.
Heretofore, LED driver integrated circuits with temperature compensation were known. A review of such circuits is given in Optical Fiber Telecommunications, Ed. S. E. Miller and I. P. Kaminow, Academic Press, 1988, pp. 727-730. However, in such circuits the LED current flows through many transistors comprising the driver and/or the temperature compensation circuitry. This means that the transistors for the driver and temperature compensation must be able to handle the large currents required for the LED. Transistors with larger currents have higher power dissipation, shorter operational lifetimes and are more expensive. The principal object of the present invention is to provide a transmitter circuit where the LED current flows only through one transistor in the driver and does not flow through the temperature compensation circuit.
An object of the present invention is to provide a transmitter circuit which can be fabricated from the metal semiconductor field effect transistor (FET) process for integrated circuit fabrication. As described in the review by Miller and Kaminow, driver circuits with temperature compensation have utilized bipolar transistors. However, the proliferation of foundries for the custom fabrication of FET integrated circuits has resulted in the need for new driver circuits which take advantage of the advantages of FET processing while minimizing their weaknesses. Among these advantages is a high reproducibility of transistors. Among the weaknesses is a low precision in resistors. The present invention of a LED driver circuit with temperature compensation maximizes the use of transistors and minimizes the use of resistors.
Another object of the present invention is to provide a transmitter circuit which is capable of being fabricated with the driver circuit and LED on the same integrated circuit substrate, also known as monolithic integration. Recent developments in semiconductor device processing methods enable the fabrication of such an integrated transmitter.